Hydrogen is supplied to customers connected to a hydrogen pipeline system. Typically, the hydrogen is manufactured by steam methane reforming in which a hydrocarbon and steam are reacted at high temperature in order to produce a synthesis gas containing hydrogen and carbon monoxide. Hydrogen is separated from the synthesis gas to produce a hydrogen product stream that is introduced into the pipeline system for distribution to customers that are connected to the pipeline system. Alternatively, hydrogen produced from the partial oxidation of a hydrocarbon can be recovered from a hydrogen rich stream. Typically, hydrogen is supplied to customers under agreements that require availability and on stream times for the steam methane reformer or hydrogen recovery plant. When a steam methane reformer is taken off-line for unplanned or extended maintenance, the result could be a violation of such agreements. Additionally, there are instances in which customer demand can exceed hydrogen production capacity of existing plants. Having a storage facility to supply back-up hydrogen to the pipeline supply is therefore desirable in connection with hydrogen pipeline operations. Considering that hydrogen production plants on average have production capacities that are roughly 50 million standard cubic feet per day or greater, a storage facility for hydrogen that would allow a plant to be taken off-line, to be effective, would need to have storage capacity in the order of 1 billion standard cubic feet or greater.
The large storage capacity can be met by means of salt caverns to store the hydrogen underground. Low purity grades of hydrogen (i.e., below 95% purity) as well as other gases have been stored in salt caverns. Salt caverns are large underground voids that are formed by adding fresh water to the underground salt, thus creating brine, which is often referred to as solution mining. Caverns are common in the gulf states of the United States where demand for hydrogen is particularly high. Such hydrogen storage has taken place where there are no purity requirements or less stringent (<96% pure) requirements placed upon the hydrogen product. In such case, the stored hydrogen from the salt cavern is simply removed from the salt cavern without further processing.
High purity (e.g., 99.99%) hydrogen storage within salt caverns presents several challenges. For example, storing large quantities (e.g., greater than 100 million standard cubic feet) of pure (e.g., 99.99%) gaseous hydrogen in underground salt caverns consisting of a minimum salt purity of 75% halite (NaCl) or greater without measurable losses is difficult based on the properties of hydrogen. Hydrogen is the smallest and lightest element within the periodic table of elements, having an atomic radius measuring 25 pm+/−5 pm. Further, hydrogen is flammable, and therefore a very dangerous chemical if not handled properly. Salt caverns consist of salt that have various ranges of permeability (e.g., 0-23×10^−6 Darcy) that if not controlled properly could easily allow gaseous hydrogen to permeate through the salt and escape to the surface of the formation. If the stored hydrogen within an underground salt formation was to escape and permeate through the salt formation to the surface, a dangerous situation could arise with fatality potential or immense structural damage potential. Consequently, high purity hydrogen is typically considered one of the most difficult elements to contain within underground salt formations.
As will be discussed, among other advantages of the present invention, an improved method and system for storing hydrogen in a salt cavern is disclosed.